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This page compares chemicals commonly available in the United States which
are intended to treat rust corrosion on steel. It is primarily directed toward the
use of such treatments on automobile sheet metal and specifically split screen vintage
(1967 and earlier) Volkswagen busses, since that is my hobby which lead me to do this
comparison.

VW bus sheet steel is formed primarily of iron. The corrosion
of iron is a complex electrochemical reaction in which the iron combines with both oxygen
and water to form iron oxides such as magnetite Fe3O4 (black rust)
and ferric oxide Fe2O3 (brown or red rust) among others. The
reaction occurs when there is a difference in the electromotive potential of metals
in the presence of a conductor. This creates a electrochemical cell
which has the following parts:

cathode: the more noble metal (less reactive on the
electromotive series)

anode: the less noble metal (more reactive on the
electromotive series)

electrolyte: conductor such as water which in the case
of iron corrosion also provides oxygen

Electrons flow from the anodic area to the cathodic area causing the metal
to corrode by forming soluble positive ions at the anode. The overall reaction
for the formation of ferric oxide:

Such an electrochemical cell can form between different metals, between
different pieces of the same metal, and even within the same piece of metal for the
following reasons:

Sheet metal is rarely pure and almost certainly contains
more noble metals.

Electrochemical cells may also form on a chemically
homogeneous metal in areas of mechanical stress, such as a dent or a bend, and concentrate
the corrosion along this stress line. Sheet metal will corrode preferentially at the point
of the bend.

Just the effects
of different oxygen concentration, temperature, and pH at a metal surface will cause
corrosion.

Even if a metal is pure without area of stress, the
presence of rain water mixed with dirt and grime (which contains traces of salts of nobler
metals) can cause the formation of local electrochemical cells at
the metal surface.

This last point is also closely related to destructive
nature of road salt. Road salt mixed with water will accelerate the
formation of rust by increasing the conductivity of the electrolyte in the cell and
specifically as follows:

At the surface of the
more noble metal (the cathode) negatively charged hydroxide ions are formed:

2H2O + 2e ® H2­ + 2(OH)-

These hydroxides combine with sodium ions from the salt water to form sodium hydroxide:

Na+ + OH-® NaOH

At the surface of the less noble metal (the anode) positively charged ferrous ions are
formed:

Fe+ - 2e ® F+2

These ferrous ions combine with chloride from the salt water to form ferrous chloride:

Fe+2 + 2CI-® FeCl2

The sodium hydroxide and ferrous chloride yield ferrous hydroxide where they meet:

FeCl2 + 2NaOH ® Fe(OH)2 + 2NaCl

In the presence of oxygen, a secondary reaction further oxidizes the ferrous hydroxide
to form hydrated ferric hydroxide (red-brown rust):

4Fe(OH)2 + O2 ® 2H2O + 2Fe2O3
× H2O

If oxygen is limited, this secondary reaction is modified to form hydrated or black
magnetite:

Fe3O4 × H2O ® H2O + Fe3O4

Of course, the subject of electrochemical corrosion of iron is vast, and this has just
highlighted the subject. Some of the information here comes from Basic
Methods of Conserving Underwater Archaeological Material Culture by Donny L.
Hamilton, Nautical Archaeology Program, Department of Anthropology, Texas A&M
University, Spring 1997.

Excluded Treatments

Paints and coatings which are intended to be applied over a rusted surface
to encapsulate the rust but not chemically change the rust, are not
included in this comparison. Some of these products are:

Electrolysis basically
reverses the electrochemical reaction which created the rust in the first place. It
does not restore the oxidized iron to its original state, but rather removes the rust from
the surface of the unrusted metal below. The iron that rusted in the first place was
an anode in the electrochemical cell. Electrolysis reverses the reaction by making
the rusted object into the cathode and some other piece of metal into the anode. An
electromotive difference is created between the cathode and anode with the use of a direct
electric currant. Just as in the original reaction which created the rust, the anode
and cathode must be bridged with an electrolyte. The anode is the positive terminal
of the electrolytic cell, to which electrons, negatively charged ions, travel when an
electric current is passed through the cell. Oxidation occurs at the anode and oxygen is
evolved. The cathode is the negative terminal of the electrolytic cell to which positively
charged metallic ions travel. At the cathode, reduction takes place and hydrogen is
evolved. For some good resources on how to do electrolysis, see http://www.bhi.co.uk/hints/rust.htm by Ted
Kinsey or http://www.oldengine.org/members/billd/electrol.htm.
Full immersion is required and rust is more effectively removed from the cathode
when the surface is in a direct "line of sight" with the anode.

Phosphoric Acid with Zinc works
like plain phosphoric acid with the addition of leaving a zinc residue on the treated
surface. The zinc is intended to protect the metal from further corrosion in the
same way galvanizing protects steel through passive cathodic protection. The iron
originally rusted since it was the anode (or more reactive) in an electrochemical cell.
Cathodic protection introduces a metal more reactive than iron (in this case zinc)
into the cell. Thus the iron becomes the cathode and the zinc becomes the
anode. The result is that the zinc will now be more prone to oxidation and corrosion
and the iron will be protected. If the conditions are right, the zinc will
eventually be consumed. However, zinc is consumed slower than iron. This is
true, even though zinc is less noble and more reactive than iron. Zinc oxide
created in the corrosion process, forms a protective barrier on the surface which prevents
further corrosion. Iron oxide formed when iron corrodes, does not form such a
protective barrier, but rather the familiar flaky and brittle rust. Products
compared in this category include:

Other Acids (besides phosphoric
acid) can be used in rust treatments including hydoxyacetic acid, sulfamic acid, and
hydrogen chloride. Hydrogen chloride (hydrochloric or muriatic acid) gives the same
two reactions as phosphoric acid with Cl in place of PO4, but in this case both
of the reactions are relatively fast, so the actual metal is lost at a rate similar
to the rust. Also, the FeCl3 produced does not adhere to the metal
surface as well, so you get little protection of the treated metal surface. Products
compared in this category include:

Conversion Coatings are
applied over the surface of rust and, unlike the other treatments in this comparison, are
left in place. These products claim to covert the underlying rust to an inert
substance and leave a black primer for further coatings. I have found little
chemistry to explain these products:

One half of a rusted steel bar was wire brushed (with an power angle
grinder) and cut into squares to produce pieces which looked like this:

This was done to compare each treatment's effect upon both "raw"
rust and rust which was has been wire brushed.

Typical TestingOther than conversion coatings, each piece was full immersed for about 30
minutes. Results were then photographed both dry and after wetting with water to
improve clarity. Each piece was then further immersed for about 2 hours and each
piece was brushed manually with a small wire brush. Results were then photographed
dry.

Conversion Coating Testing
For conversion coatings, two coats were applied. Half of the finished surface was
scrapped to reveal the condition of the rust below. This scrapping was done
perpendicular to the original division of "raw" and wire brushed rust, to reveal
the condition of the rust both in the "raw" and wire brushed areas.

Click on this picture of
testing underway to view full size

Click on this picture of
final results of all products to see full size.

Comparison Results and Organization

A large table is used to organize information for each of the products
compared. The products are listed across the columns of this table.
The following information is listed down each of the rows:

Product Name
Manufacturer / Source
Cost
Cost per oz
Active Ingredients (CAS numbers are included for reference elsewhere)
Characteristics (pH and specific gravity)Other Information (including manufacturer's description)Product Name (repeated for reference)Results after short treatment and no brushing- photographed dryResults after short treatment and no brushing - photographed wetResults after long treatment and brushing - photographed dryProduct Name (repeated for reference)Untreated Comparison (right side was wire brushed before
treatment; left side was left "raw")

My apologies, but the reader will need to do a bit of scrolling to see all the
information.

Manufacturer's
Description: MP-7 is designed to dissolve rusts and oxides from metal surfaces.
Chemically, it reverses the rusting reaction and cleans away all oxidation from the metal.
Also, minute crystals are formed that become integral with the surface molecules of the
base metal, inhibiting further oxidation. These deposits are completely inert and do not
change the tolerances or the conductivity of the metal. After the rust is dissolved and
MP-7 has dried, the surface is ready for the next step, whether it be painting or welding

Manufacturer's
Description: a serious, industrial-strength chemical, which, because it is
chemically buffered, is safe, fumeless, and harmless to your kin when used according to
all directions and your common sense

Recommended by Thom
Fitzpatrick

Manufacturer's Description: As seen on TV

Per John S. Henry johnshenry@sprintmail.com, liquid better than
aerosol. Converts rust to black, inert substance which includes magnetite (Fe3O4)

Manufacturer's
Description: Completely passivates the rust, chemically preventing it from
participating in further corrosion. Each molecule of RUST-X can attract and incorporate
several iron atoms, and each single iron atom can be linked to three different molecules
within RUST-X. The result is that RUST-X envelopes the rust at the molecular level,
developing into a neutral layer which exists between the steel surface and the atmosphere.
The neutral layer prevents the migration of ions so that no chemical reaction can occur.
Dispersible in water

Comments

Product Name

electrolysis

Ecotec

Simple Solutions Rust Remover

Rust-Mort

MP-7

Naval Jelly

Dupont 5717S

Mirachem 250

Dupont 5718S

Metal-Ready

Envirosolve 1000

CLR - Calcium Lime & Rust
Remover

Extend

One-Step

Rust-X

Product Name

Results after short treatment and no
brushing- photographed dry

no
picture

Results after short treatment and no
brushing- photographed dry

Results after short treatment and no
brushing - photographed wet

no
picture

Results after short treatment and no
brushing - photographed wet

Results after long treatment and
brushing - photographed dry

no picture

long duration treatment
not applicable
to conversion treatments

long duration treatment
not applicable
to conversion treatments

long duration treatment
not applicable
to conversion treatments

Results after long treatment and
brushing - photographed dry

Product Name

electrolysis

Ecotec

Simple Solutions Rust Remover

Rust-Mort

MP-7

Naval Jelly

Dupont 5717S

Mirachem 250

Dupont 5718S

Metal-Ready

Envirosolve 1000

CLR - Calcium Lime & Rust
Remover

Extend

One-Step

Rust-X

Product Name

Untreated Comparison - right side was wire
brushed before treatment; left side was left "raw"

Untreated Comparison - right side was wire
brushed before treatment; left side was left "raw"

My
Conclusions

The following are my conclusions and recommendations for treating rust that I use in my
shop (no powdercoating, plating, or galvanizing facilities). These are listed in
order of my preference:

1Sandblast - Sandblasting is
by far the best method for treating rusted metal. It leaves the metal very clean,
although some micro rust my be left embedded deep in the surface. It is also very
easy. There are some drawbacks including:

The surface has
to be accessible to the blasting nozzle (not inside a door panel, or an inaccessible
cranny for example).

The abrasive is
very intrusive and will get everywhere.

A wire brush
(either power or hand) will compliment any of the rust treatments, but will won't get all
the rust.

2Electrolysis
- Electrolysis seemed to work just as well if not better than other chemical treatments,
and since it is so inexpensive, it may be the way to go. There are drawbacks here
also:

The part must be full immersed, which can be a problem with large objects.

The surfaces you want to remove the rust from must be in direct line-of-sight with the
anodes (again won't get inside a door panel unless you can put an anode in there without
shorting the electrolyte).

I have tried electrolysis since conducting this comparison, and have ended up with a
layer of black magnetite on the surface. This is easily brushed off, but makes me
wonder if I have something wrong or why sandblasting is not just as easy.

3Friendly Chemicals
- When I need to use acid treatments, I like to use the Ecotec. It is a little more
expensive, but it works very well and is friendly to use and dispose of. In my
experience, Simple Solutions is not a trustworthy business.

4Plain Phosphoric
Acid Based treatments - These all seemed to work pretty well. Naval Jelly
is useful where a jell is needed to cling to surfaces. Dupont 5717S comes in a
concentrated solution which should be diluted according to the manufacturer's directions.
I have used 5717S undiluted on though rust and it works very well, but it is nasty
to work with.

5Phosphoric Acid
with Zinc - I would not use these as a rust treatment alone, since the
concentration of phosphoric acid is less than the plain phosphoric acid based treatments.
However, when used after treatment with the plain phosphoric acid based chemicals,
the zinc in these treatments may do some good. The scary part is that directions
indicate that the parts be cleaned with water after treatment. Cleaning raw steel
with water is not something that thrills me. The steel seems to oxidize immediately,
but this might just be the zinc that is oxidizing.

6Other Acids -CLR seems to work okay and is pretty inexpensive. I prefer the phosphoric
acid since it leaves the iron phosphate residue. I found no benefit for the
Envirosolve 1000 (expensive and no residue).

7Conversion Coatings
- I do not "believe" in these treatments, and you must have faith to believe the
manufacturer's claims. They are supposed to "convert" the rust, but after
using them, I have scrapped them off and found rust still there underneath. If you
think about it, these products dry in a matter of minutes. How can this provide
enough time for the product to penetrate into and convert the rust?

Nothing on this page should be construed as following scientific method. These
comparisons were done pretty much off the cuff in my shop. Any conclusions or
recommendations are purely my subjective opinion.